The web templates connector has not changed in many years, so I am doubtful that your issues is because you are on Kepler.
Do you see the section for Advanced Configuration in the query editor? You have to expand the section to get the preview button etc.
Can you post a screenshot of what the wizard looks like for you?
Can you tell me what version of org.eclipse.mylyn.web.tasks is installed in your Eclipse?

That is the Task Repository configuration, the preview button is a part of the Repository Query configuration.
In Mylyn you define a repository, then you create queries which retrieve tasks from the repository, one may have none, one or many queries for a repository.